This invention relates to apparatus for melting heat-softenable materials and more particularly to steam feeders or bushings made of platinum, rhodium or other precious metal alloys for producing continuous glass fibers and to a method of fabricating steam feeders from such metals.
Several methods have heretofore been employed in processing glass for forming continuous glass filaments or fibers. One method involves the steps of melting glass in a comparatively large furnace, refining the glass in a refining chamber, and forming the glasss into spherical bodies or marbles. The glass marbles are subsequently delivered into a stream feeder which is electrically heated to remelt the glass to a viscosity at which the streams of glass may be flowed through orifices in the feeder and attenuated into fibers.
The more common method used today is the direct melt process wherein glass batch is reduced to a molten state and refined in a furnace. The molten glass flows from the furnace along a forehearth channel to stream feeders disposed along the forehearth. The feeders are heated by electrical resistance heating to control glass viscosity. And the streams of the glass are delivered through orifices in the feeders or bushings for attenuation into fibers.
Both of the above fiber forming processes employ stream feeders or bushings made of high temperature resistant metal alloys such as platinum or rhodium. The steam feeder is a long metal box having orificed tips or projections attached to its bottom wall through which streams of glass flow for attenuation into fibers. Terminals to which electrical bus bars are attached for supply of current through the feeder are welded to either end of the box-shaped feeder. The feeder is then heated by its own electrical resistance. To achieve the desired heating characteristics in the feeder, it has conventionally been the practice to reduce the thickness of the feeder walls relative to the orifice plate to control the amount of current passing therethrough.
Feeders have typically been manufactured from precut parts which are welded together by conventional fusion welding techniques. The plate for the bottom wall or tip section is usually of uniform thickness and the plates for the sides and flanges are of various lesser thicknesses to produce the desired heat pattern and/or to reduce the amount of precious metal required. Using thinner pieces for the side walls and flanges causes several difficulties in conventional feeder fabrication as well as feeder performance. For example, additional rolling of the thinner sections is required. The welding together of the pieces is time consuming and the uniformity of the welds may vary. Also, because of the cast nature of the fusion bead as compared to the wrought sheet, resistivity is changed through the weld zone and the same heat pattern may be difficult to stabilize or reproduce from feeder to feeder. Severe warpage of the thinner sheet material may occur when fusion welded to the thicker material. In addition, if the welds are not made extremely carefully, they may leak glass. Also, if failures or fractures occur in the feeders, they usually occur in or adjacent to the welds.
If a technique could be developed for reducing the adverse effects of fusion welding during construction of the feeders or for improving current distribution through the feeder when it is in operation, a substantial contribution would be made to advance the art of forming high-temperature continuous fibers.